We always want there to be an answer, except there never is. Something like Virginia Tech happens, people want to know why.

Jack Levin, the director of the Brudnick Center on Violence and conflict at Northeastern University in Boston, author of more than two dozen books on murder and criminology summed it up. "We're still in the dark about where this comes from."

He co-wrote "Mass Murder: America's Growing Menace," in 1985. Since then there have been lots of books about serial killers, lots of brain research and many more mass shootings. There are MRIs and talk about high levels of neurotransmitters like dopamine and plunging levels of serotonin. There's research into the limbic system, a primitive part of the brain that controls emotions and behavior. New medications have revolutionized psychiatric care for depression, even psychotics.

None of it really touches the psychology of mass murder.

"In mass shootings, the killer is often killed themselves, so we don't really have the ability to interview and analyze them — all you can really do is work off their behavior," said Neil Kaye, an assistant professor of psychiatry and human behavior at Thomas Jefferson University in Philadelphia. "The problem with that is that mass killers do this for multiple reasons, and even when you develop a profile of people at risk, 99 percent of them never go out and do anything bad."

Some of the research tells us the obvious: About 95 percent of mass killers are men, they tend to be loners, they feel alienated. They look normal on the outside and are really, really angry inside.

And yet, there are some minor lessons to be learned from this grotesque taxonomy.

Mass murder — like Monday's — is starkly different from serial killings, the other type of murder that fascinates and frightens.

Serial killers, forensic psychiatrists say, derive sexual gratification from their killings. The Ted Bundys, the Jeffrey Dahmers, the John Wayne Gacys — they don't want to be caught. They often enjoy taunting police. The violence is, in its own perverse way, about pleasure.

"Serial killers are more like drug addicts than anything else," Kaye said. "They need to ramp up the excitement each time, they're getting reinforcement from their acts. They're running on the dopamine side of the brain. They're running on highs."

It's not that way for mass killers — guys who take out a gun and try to kill as many people as possible. They're not looking for highs — they're depressed, angry and humiliated. They tend to be rejected in some romantic relationship, or are sexually incompetent, are paranoid, and their resentment builds. They develop shooting fantasies for months or years, stockpiling dreams and ammunition. The event that finally sets them off, Welner says, is usually anticlimactic — an argument, a small personal loss that magnifies a sense of catastrophic failure.

"But they don't 'snap,' as you so often hear people say," Welner said. "It's more like a hinge swings open, and all this anger comes out."

They plan everything about the killings, he says, except how to get away.

Psychosis is common among perpetrators of mass shootings as a whole, he says, but less common among shooters who are well organized and therefore kill more victims. Psychotics hear voices and people from outer space and talking dogs.

Perhaps these sorts of taxonomies are the building blocks of actual knowledge, and someday they will matter.

For now, there are no real answers, no real solace, no real consolation.